Polymeric optical films are used in a wide variety of applications. Particular uses of polymeric films include mirrors and polarizers. Such reflective films are used, for example, in conjunction with backlights in liquid crystal displays. A polarizing film can be placed between the user and the backlight to recycle polarized light that would be otherwise absorbed, and thereby increasing brightness. A mirror film can be placed behind the backlight to reflect light towards the user; thereby enhancing brightness. These polymeric optical films often have extremely high reflectivity, while being lightweight and resistant to breakage. Thus, the films are suited for use as reflectors and polarizers in compact electronic displays, such as liquid crystal displays (LCDs) placed in mobile telephones, personal data assistants, portable computers, desktop monitors, and televisions. Another application of these polarizing films includes, for example, solar control.
One class of polymers useful in creating polarizer or mirror films is polyesters, described in U.S. Pat. Nos. 5,825,543 and 5,867,316 and PCT Publications WO 99/36262 and WO 97/32226, incorporated herein by reference. One example of a polyester-based polarizer includes a stack of polyester layers of differing composition. One configuration of this stack of layers includes a first set of birefringent layers and a second set of layers with an isotropic index of refraction. The second set of layers alternates with the birefringent layers to form a series of interfaces for reflecting light.
Although polymeric optical films can have favorable optical and physical properties, one limitation with some such films can be dimensional instability of the film when exposed to substantial fluctuations in temperature. This dimensional instability can result in formation of wrinkles in the film as it expands and contracts. Such dimensional instability can be particularly common when temperatures approach or exceed approximately 80° C. Warping also may be observed when some films are cycled to high temperatures and high humidity conditions, such as conditions of 60° C. and 70 percent relative humidity.
Another limitation of some polymeric optical films is that they fail to dissipate static charges. Static charges on a polymeric optical film can be detrimental to the assembly of optical devices including such films, and can cause static attraction to other films or glass in the backlight display. The static attraction to other films or glass in the backlight display may sometimes cause visual defects manifesting themselves as circular shadows or variations in brightness. In addition, many static dissipative materials are not compatible with polymeric optical film materials.
Another limitation of some polymeric optical films is their tendency to degrade when exposed for long periods of time to the UV light from fluorescent bulbs in backlight displays. These optical films can become undesirably yellow from UV light-induced degradation.